1. Field of the Invention
The present invention relates to an optical transmission system, and, in particular, is preferably used in a reproduction interconnection device in optical communication.
2. Description of the Related Art
In optical communication, a reproduction interconnection device is used in order to suppress deterioration of an optical signal due to a noise of an amplifier or waveform distortion during transmission. When an optical nonlinear element which has nonlinear input/output characteristics as shown in FIG. 7 is used for the reproduction interconnection device, fluctuation (noise) of level “0” and level “1” of the optical signal due to waveform deterioration associated with transmission can be compressed. As for such an optical nonlinear element in which a relation of an input power and an output power is nonlinear, numerous documents are disclosed, such as Patent Document 1 and Patent Document 2, for example.
In a case that waveform-shaping of the optical signal is performed using the optical nonlinear element, if an input signal is the one having a specific input power (for example, an input power IP2) on an input/output curve of the optical nonlinear element as shown in FIG. 8, reproduction is properly performed and an output signal WVO2 with a good waveform can be obtained. However, if the power of the input signal deviates from the specific input power, reproduction may not be properly performed and the waveform may not be improved.
As shown in FIG. 8, if the power of the input signal is too low (for example, an input power IP1), the noise of level “0” in the optical signal is compressed by the optical nonlinear element, but the noise of level “1” is emphasized (see an output signal WVO1). On the other hand, if the power of the input signal is too high (for example, an input power IP3), the noise of level “1” in the optical signal is compressed by the optical nonlinear element, but the noise of level “0” is emphasized (see an output signal WVO3).
In the reproduction interconnection device in which the optical nonlinear element is used, a condition (input power to the optical nonlinear element) enabling to obtain an output signal of a good waveform is determined in response to the input signal and the input/output characteristics of the optical nonlinear element. When the condition is to be determined, it is necessary to recognize a cross point in an eye pattern of the input signal or the output signal. Here, in Patent Document 3, Patent Document 4 and the like, evaluation methods of optical signals are proposed.
[Patent Document 1] Japanese Patent Application Laid-open No. 2001-222037
[Patent Document 2] Japanese Patent Application Laid-open No. 2002-44020
[Patent Document 3] Japanese Patent Application Laid-open No. 2004-173026
[Patent Document 4] Japanese Patent No. 3459213
However, it is not easy to directly measure the cross point in the input signal or the output signal to determine the input power to the optical nonlinear element. Additionally, kinds of the input signal inputted to the optical nonlinear element vary. Therefore, it is difficult to respond to various input signals by a method in which the cross point is directly measured to control the input power to the optical nonlinear element to be a specific value enabling to obtain a good output waveform.